1. Field of the Invention
The invention is directed to a circuit for generating an alternating voltage from a plurality of unregulated voltage sources with an output DC voltage that is variable over time, and a method for triggering the circuit. Photovoltaic arrays or parts of such arrays are preferred as such unregulated voltage sources. The term photovoltaic array should be understood here to mean solely the configuration and electrical connection of a plurality of photovoltaic modules. Such photovoltaic modules are connected to make photovoltaic arrays and have a structurally dictated output DC voltage that is dependent on the amount of incident sunlight and that is accordingly slowly variable over time.
2. Description of the Related Art
To feed the output current of a photovoltaic array such as described into the public power grid or a local power grid, the output DC voltage of the array must be converted by means of an inverter circuit into an alternating voltage of constant frequency and voltage.
One example of a circuit arrangement for generating such an alternating voltage is described in German patent disclosure DE 10 2008 034 955 A1. That disclosure describes a current converter having a photovoltaic array with an output voltage that is temporally variable, a level converter, and an inverter. In the associated method, first and second capacitors of the level converter are each charged with half the value of the desired intermediate circuit voltage, regardless of the input voltage.
Many demands are made of such circuit arrangements, depending on the intended use, and some of these demands are contradictory. For example, the input voltage at the inverter circuit should be as constant as possible, and the transmission from the photovoltaic array to the inverter circuit should be done at as high a voltage as possible to keep line losses low. The entire circuit should also operate at high efficiency. At the same time, the photovoltaic array, or its photovoltaic modules, should be operated in such a way that it operates at the MPP (maximum power point). This requires suitable regulation, known as MPP tracking.